Reciprocal helical scanning is recently available as one of the scanning schemes using an X-ray computed tomography apparatus (to be referred to as an X-ray CT apparatus hereinafter). Reciprocal helical scanning is an imaging technique of continuously reciprocating the top while continuously rotating the X-ray tube on a circular orbit centered on an object to be examined. With regard to the reciprocal movement of the top, imaging along a forward path will be referred to as forward scanning hereinafter. With regard to the reciprocal movement of the top, imaging along a backward path will be referred to as backward scanning.
According to reciprocal helical scanning, the X-ray tube (or the X-ray detector) traces a locus in a helical form (to be referred to as a helical locus hereinafter) with respect to an object. This will obtain a tomographic image with excellent continuity in a wide range. For example, reciprocal helical scanning on an object injected with a contrast medium is used to analyze hemodynamics (perfusion).
Conventional reciprocal helical scanning, however, has the following problems. First, the object is not imaged during a returning process from forward (backward) scanning to backward (forward) scanning, i.e., an acceleration/deceleration process of the top (for example, FIG. 19). An imaging wait time during an acceleration/deceleration process of the top degrades the temporal resolution in perfusion analysis. Second, X-rays are emitted when the top reaches a constant velocity while the rotating frame on which the X-ray tube and the X-ray detector are mounted is rotated in advance at a predetermined angular velocity. For this reason, different helical loci are sometimes traced in different forward (backward) scans (for example, FIG. 20). The differences between helical loci in different forward (backward) scans cause differences in image quality (to be referred to as image quality differences hereinafter) concerning the same imaging position in the object. If the temporal resolution is low and image quality differences occur at the same imaging position, perfusion analysis sometimes becomes inaccurate.